Prevent crash from set_filter before GUI created

Also adds a remote call `get_active_filter` to return a player's current active filter without changing it.

Ref: #109

Author: narc0tiq

changelog.txt

@@ -1,4 +1,11 @@
 ---------------------------------------------------------------------------------------------------
+Version: 0.8.13
+Date: 2019-05-06
+  Bugfixes:
+    - Prevent crashing from trying to set the active filter before the YARM GUI is created.
+  Features:
+    - Add a remote call `get_active_filter(player_name_or_index)` that returns the given player's active filter without changing it (avoiding the need to call `set_filter` twice).
+---------------------------------------------------------------------------------------------------
 Version: 0.8.12
 Date: 2019-05-02
   Bugfixes:

remote.lua

@@ -24,6 +24,13 @@ function interface.reset_player(player_name_or_index)
     player_data.remote_viewer = nil
 end
 
+function interface.get_current_filter(player_name_or_index)
+    local player = game.players[player_name_or_index]
+    local player_data = global.player_data[player.index]
+
+    return player_data.active_filter or 'none'
+end
+
 function interface.set_filter(player_name_or_index, new_filter)
     local player = game.players[player_name_or_index]
     local player_data = global.player_data[player.index]

resmon.lua

@@ -819,7 +819,12 @@ end
 
 
 function resmon.update_ui_filter_buttons(player, active_filter)
-    local buttons_container = mod_gui.get_frame_flow(player).YARM_root.buttons
+    -- rarely, it might be possible to arrive here before the YARM GUI gets created
+    local root = mod_gui.get_frame_flow(player).YARM_root
+    -- in that case, leave it for a later update_ui call.
+    if not root or not root.valid then return end
+
+    local buttons_container = root.buttons
     for filter_name, _ in pairs(resmon.filters) do
         local is_active_filter = filter_name == active_filter